This invention relates generally to acoustic well logging of formations surrounding a borehole, and more particularly to methods and apparatus for generating and receiving high energy acoustic signals.
It is common in the oil and gas industry to traverse a borehole with an acoustic logging instrument for obtaining selected measurements relating to the producibility of any oil or gas which may be present in the formations surrounding the borehole. As the acoustic logging instrument traverses a borehole high energy acoustic signals are generated by an acoustic transducer functioning as a transmitter. These high energy acoustic signals travel through a portion of the surrounding earth formations and are detected by an acoustic transducer functioning as a receiver located a longitudinally spaced distance from the transmitter. One common measurement derived by the acoustic logging instrument is the acoustic energy travel time from the location of acoustic energy generation to the location of receipt by the receiver. This interval travel time is the reciprocal of the acoustic energy velocity through the interval. Such a measurement can provide formation information related to porosity, fracturing and lithology.
One form of acoustic transducer often used in well logging applications is the magnetostrictive transducer. The magnetostrictive acoustic transducer typically is constructed of a thin layer of ferromagnetic material wrapped about itself to form a hollow cylinder. A large number of turns of wire are wound toroidally about the cylinder. A high-amplitude pulse of relatively short duration is applied to the winding causing the establishment of a magnetic field. The presence of the magnetic field causes a mechanical deformation of the ferromagnetic core resulting in energy being emitted from the transducer. The magnetostrictive transducer, although durable, is limited in energy output due to saturation of the ferromagnetic core. Additionally, due to the relatively high impedance of the winding it is difficult to produce the relatively short output pulse required for acoustic logging.
Yet another form of acoustic energy transducer used in acoustic well logging applications is the piezoelectric transducer. The piezoelectric transducer is constructed of a ceramic cylinder to which a short-time duration, high-amplitude pulse is applied to produce an output. The change in the applied signal results in a change on the stress of the ceramic material thereby causing an impulse of energy to be emitted by the transducer. For applications in acoustic well logging, the piezoelectric transducer is extremely fragile. Additionally, the ceramic cylinder has a relatively low limit of internal stress which results in shattering if the pulse applied thereto is too large in amplitude.
These and other disadvantages are overcome with the present invention by providing a method and an apparatus for generating and receiving acoustic energy which consists of a plurality of bonded concentric cylinders of ferromagnetic material through which a pulse of current is passed and a torrodial winding about the cylinders coupled to a magnetization source.